The present invention is directed to polymerization processes and more directly related to an initiation system for controlled radical polymerization processes.
ATRP is one of the most successful controlled/xe2x80x9clivingxe2x80x9d radical processes (CRP) developed and has been thoroughly described in a series of co-assigned U.S. Patents and Applications, U.S. Pat. Nos. 5,763,546; 5,807,937; 5,789,487; 5,945,491; 6,111,022; 6,121,371; 6,124,411; 6,162,882; 6,407,187; and U.S. patent application Ser. Nos. 09/018,554; 09/359,359; 09/359,591; 09/369,157; 09/534,827; 09/972,046; 09/972,056; 09/972,260; 10/034,908; and 10/098,052 all of which are herein incorporated by reference, and has been discussed in numerous publications by Matyjaszewski as co-author and reviewed in several publications.
A living polymerization process is a chain growth process without or with an insignificant amount of chain breaking reactions, such as transfer and termination reactions. Controlled/living polymerization, herein xe2x80x9ccontrolled polymerizationxe2x80x9d, is a chain growth process that under controlled polymerization conditions provides effective control over the chain growth process to enable synthesis of polymers with molecular weight control and narrow polydispersities or molecular weight distributions. Molecular weight control is provided by a process having a substantially linear growth in molecular weight of the polymer with monomer conversion accompanied by essentially linear semilogarithmic kinetic plots, in spite of any occurring terminations. Polymers from controlled polymerization processes typically have molecular weight distributions, characterized by the polydispersity index (xe2x80x9cPDIxe2x80x9d), of less than or equal to 2. The PDI is defined by the ratio of the weight average molecular weight to the number average molecular weight, Mw/Mn. More preferably in certain applications, polymers produced by controlled polymerization processes have a PDI of less than 1.5, and in certain embodiments, a PDI of less than 1.3 may be achieved.
Polymerization processes performed under controlled polymerizations conditions achieve these properties by consuming the initiator early in the polymerization process and, in at least one embodiment of controlled polymerization, an exchange between an active growing chain and dormant polymer chain is equivalent to or faster than the propagation of the polymer. A controlled radical polymerization (xe2x80x9cCRPxe2x80x9d) process is a process performed under controlled polymerization conditions with a chain growth process by a radical mechanism, such as, but not limited to, atom transfer radical polymerization, stable free radical polymerization, specifically, nitroxide mediated polymerization, reversible addition-fragmentation transfer/degenerative transfer/catalytic chain transfer radical systems. A feature of controlled radical polymerizations is the existence of an equilibrium between active and dormant species. The exchange between the active and dormant species provides a slow chain growth relative to conventional radical polymerization, but all polymer chains grow at the same rate. Typically, the concentration of radicals is maintained low enough to minimize termination reactions. This exchange, under appropriate conditions, also allows the quantitative initiation early in the process necessary for synthesizing polymers with special architecture and functionality. CRP processes may not eliminate the chain breaking reactions, however, the chain breaking reactions are significantly reduced from conventional polymerization processes.
Polymers produced under controlled polymerization conditions have a degree of polymerization that may be determined from the ratio of the amount of consumed monomer to the initiator, a polydispersity close to a Poisson distribution and functionalized chain ends. The level of control attained in a particular polymerization process is typically monitored by analyzing the kinetics of the polymerizations, the evolution of molecular weights, polydispersities and functionalities with conversion.
The equilibrium required for ATRP controlled polymerization processes has been attained using two different initiation methods or activation reactions called respectively, normal and reverse ATRP. See, for example, U.S. Pat. No. 5,763,548.
Unless otherwise indicated, all numbers expressing quantities of ingredients, time, temperatures, and so forth used in the present specification and claims are to be understood as being modified in all instances by the term xe2x80x9cabout.xe2x80x9d Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, may inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
Normal ATRP Initiation
Normal ATRP processes are initiated by the redox reaction between a initiator comprising a transferable atom or group and catalyst comprising a transition metal complex in a lower oxidation state. A redox reaction occurs between the initiator and the transition metal complex. The transferable atom or group is a group that may be homolytically cleaved from an initiator by the catalyst, thereby oxidizing the catalyst to a higher oxidation state and forming a radical thereby activating the initiator residue for monomer addition. After initiation, an ATRP process, generally, is based a dynamic equilibrium between a transition metal complex reversibly activating and deactivating the polymer chain via a similar homolytic atom or group transfer via a redox reaction. Subsequent to monomer addition, the polymer chain is activated by the removal of a transferable atom or group from the end of the polymer chain and may then deactivated by return of a transferable atom or group in the reverse reaction, the returning atom or group may not necessarily be the same atom or group removed in the activating step or even from the same transition metal complex. The equilibrium between the growing and dormant chains allows the synthesis of well-defined polymers with complex architecture. During the dynamic equilibrium the transition metal complex cycles between a lower oxidation state and a higher oxidation state. The advantages of normal initiation of ATRP include that the initiator includes the transferable atom or group needed to terminate each polymer chain, therefore no additional transferable atoms or groups are required to be added by other components of the polymerization process in order to attain polymers with the desired degree of polymerization at high conversion of monomer(s) to polymer. Therefore, only enough transition metal complex in the lower oxidation state is needed to provide suitable catalytic activity to the process. By suitable catalytic activity, it is meant that the polymerization comprises an amount of catalyst needed to drive the reaction to a desired degree of polymerization in a time that allows appropriate heat control to allow for a controlled reaction. The disadvantages of normal initiation of ATRP are that the transition metal complex in the lower oxidation state is less stable than the transition metal complex in the higher oxidation state and, typically without special handling procedures, has to be prepared at the time of reaction or stored under an inert atmosphere. Further, care has to be taken with the other reagents in the reaction to reduce the level of oxidants in the system to retain an active catalyst system, since if such termination reactions occur, the amount of catalyst in the lower oxidation state may be reduced, thereby also reducing the rate of polymerization.
Any transition metal complex capable of maintaining the dynamic equilibrium with the polymer chain may be used as the redox catalyst in ATRP, as discussed in the cited art, after consideration of oxidation states, complex formation with suitable ligands and redox potential of the resulting complex to provide a catalyst for the desired reaction. A wide variety of ligands have been developed to prepare transition metal catalyst complexes that display differing solubility, stability and activity.
The embodiments of the present invention described herein exemplify the process by focusing on the use of copper as the transition metal and butyl acrylate as the monomer but as described in the cited art other transition metals can be used and a wide range of radically (co)polymerizable monomers can be used. Any radically (co)polymerizable monomer can be polymerized by preparing a catalyst complex with appropriate solubility, stability and activity as taught in the cited references and the relevant art.
Reverse ATRP Initiation
In reverse ATRP, as opposed to normal ATRP, the transferable atom or group begins as a counterion or ligand on the transition metal complex in the higher oxidation state. Generally, the higher oxidation state of a transition metal complex is an oxidatively more stable state of the complex. The reaction is then initiated by a generation of a radical by known processes, such as, by decomposition of a standard free radical initiator which may participate in a redox reaction with the transition metal forming the transition metal complex in the lower oxidation state, the activator state, and a molecule with a transferable atom suitable for initiation of an ATRP reaction. In this regard, a reverse ATRP process allows for controlled polymerization starting from lower cost, more oxidatively stable Cu(II) complexes, however, reverse ATRP systems require high catalyst concentration in order to introduce the appropriate concentration of radically transferable atoms or groups to the reaction in order to maintain a controlled polymerization and to attain polymers of the desired molecular weight at high conversion of monomer to polymer. Due to the high concentration of catalysts required in reverse ATRP processes, only transition metal complexes with lower catalytic activity, such as, catalysts comprising bipyridine type ligands have been successfully employed. As used herein, catalysts employed in ATRP polymerizations resulting a rate of polymerization greater than an ATRP polymerization comprising a bipyridine ligand under similar conditions is considered highly active, preferable the rate of reaction is 1.5 times the rate of the reaction using a bipyridine ligand under similar conditions.
A standard free radical initiator is essentially the source of free radicals required to initiate the polymerization of the radically polymerizable monomers. The free radicals may be formed by thermal or photoinduced decomposition of the initiator or by a redox reaction with the initiator. Typical standard free radical initiators include, but are not limited to, dialkyl diazenes, including, azobis(isobutyronitrile) (xe2x80x9cAIBNxe2x80x9d), dimethyl 2,2xe2x80x2-azobisisobutyrate (MAIB), 1,1xe2x80x2-azobis(1-cylcohexanenitrile), 2,2xe2x80x2-azobis(2,4,4-trimethylpentane), and azobis-2,4-dimethylvaleronitrile, polymeric or oligomeric materials comprising azo, xe2x80x94Nxe2x95x90Nxe2x80x94, groups, peroxides such as acyl and diacyl peroxides, alkyl peroxides, dialkyl peroxydicarbonates, hydroperoxides, peresters, and inorganic peroxides, benzoyl peroxide (BPO) or a peroxy acid such as peroxyacetic acid or peroxybenzoic acid, styrenes and acrylates. Standard commercial free radical initiators, such as V-044, that initiate radical polymerizations are typically generated by thermal or photochemical homolytic cleavage of covalent bonds to form the radicals.
Because a typical reverse ATRP process depends on the thermal decomposition of the standard free radical initiator, a further limitation of a reverse ATRP process has been the narrow operating temperature range to ensure fast decomposition and rapid initiation of the process at low conversion of monomer to polymer to provide polymers with narrow molecular weight distribution.
ATRP catalysts vary in catalytic activity based upon the properties of the transition metal, the ligands and the temperature and polarity of the reaction medium, as well as other factors. More active catalytic systems are generally less oxidatively stable in their lower oxidation states, such as the Cu(I) complexes discussed herein to exemplify the processes. Such catalysts in their lower oxidation states may create handling problems. For instance, trace levels of oxygen should be to be removed from the system prior to addition of the catalyst in a lower oxidation state and the catalyst complex may not easily be prepared in advance of the polymerization process. Reverse initiation of ATRP, using more stable Cu(II) complexes in the initiating step, may be a convenient method to circumvent these handling problems. The preparation of oxidatively stable, active catalyst precursors, will allow larger scale manufacture, storage and shipment of catalyst systems.
Unfortunately, the known reverse ATRP initiation methods are difficult to apply to controlled polymerization using highly active catalysts such as CuBr/Me6-TREN for the preparation of lower molecular weight polymers at lower temperatures, or lower degrees of polymerization. The concentration of the catalyst to be added to a reverse ATRP is also related to the amount of added initiator and the temperature at which the initiator undergoes rapid decomposition. Additionally, rapid decomposition of the initiator is required to ensure each polymer chain grows simultaneously to synthesize polymers with narrow molecular weight distribution. When using highly active catalysts in normal ATRP reactions the catalyst concentration is low, for instance, the concentration of highly active catalysts may be less than 10% of that used with lower activity ligands and may be added at molar concentrations less than the initiator. Highly active catalysts may additionally allow a reduction in the reaction temperature without reducing polymerization rate. See Table 1, describes a bulk
polymerization of acrylates using an active catalyst at 23xc2x0 C. The polymerization process was conducted twice, at initial molar concentration ratios of catalyst to initiator of 1.0 and 0.1. Both polymerizations resulted the synthesis of polymers with low molecular weight distributions.
A reverse ATRP using AIBN as an initiator would require an initial molar concentration ratio of catalyst to initiator of approximately 1.6 and an operating temperature of over 100xc2x0 C. to result in similar polymers. The net result of increased catalytic activity is that an ATRP reaction may be driven to higher conversion and synthesize polymers displaying narrower MWD while employing less transition metal in the system and operating at a lower temperature.
However, as indicated above, when a highly active catalyst is used in a reverse ATRP, the polymerization is not easily controlled. See Table 2 for the initial concentrations of the polymerization conducted at 90xc2x0 C.
Since the amount of catalyst added to the system is dependent on the amount of initiator present and not the activity of the catalyst, an excessive amount of catalyst is required, and the resulting polymerization is uncontrolled. To supply a sufficient quantity of transferable atom or group to the polymerization the initial concentration of catalyst was determined from Equation 1, assuming a standard free radical initiator efficiency of 80%.
[Cu(II)]0/[AIBN]0=2*0.8 Equation 1 Reverse ATRP under these conditions, resulted polymerization with polymers having a nonlinear increase in molecular weight versus monomer conversion and though the molecular weight distribution was low, 1.6, the molecular weight distribution increased with monomer conversion. See FIG. 1.
However, an advantage of reverse initiation of an ATRP process is that the catalyst may be supplied in its more stable higher oxidation state. One disadvantage of reverse initiation of ATRP is that the amount of initiator in the process is dependent of the amount of transition metal complex in the higher oxidation state. The activity of the formed transition metal complex is not of primary concern, therefore more active catalysts may not provide a controlled polymerization at high ratios of catalyst to monomer.
The present invention is related to a polymerization process comprising a dual initiation system for atom transfer radical polymerization (xe2x80x9cATRPxe2x80x9d). The initiation system comprises both standard free radical initiators and initiators comprising a transferable atom or group. In certain embodiments the initiation system of the present invention comprises initiation of a reverse ATRP and normal ATRP process. The dual initiation system may be used to prepare any type of polymer that may be prepared by ATRP, such as, but not limited to, homopolymers, graft, branched, star, comb, bottle brush, block copolymers, gradient, alternating, as well as other polymer structures. Additionally, the dual initiation system may be utilized in atom transfer radical polymerization processes conducted in bulk, in solution, in emulsion, in miniemulsion, and in heterogeneous polymerizations from surfaces.